A time-domain fatigue damage assessment approach for the tripod structure of offshore wind turbines

Welded features in the tubular joints are one of the most severe sources of fatigue failure in supporting structure for offshore wind turbines (OWTs). Therefore, fatigue resistance becomes a vital design, inspection and maintenance criterion for tripod supporting structures. This paper proposes a time-domain fatigue damage assessment approach for tripod structures by establishing a detailed finite element model for the critical zones and simplified beam model for the remaining regions. The stress influence matrix (SIM) is formed once by utilizing multiple finite element analyses. Based on the equivalent static loads and linear superposition principle, the time-variant stress can be efficiently calculated. By comparing the fatigue damage caused by multiple loads to that caused by a single load in a particular direction, we conclude that the advantage of the proposed method over the conventional stress concentration factors method. Afterwards, we investigate the effects of environmental conditions on the cumulative fatigue damage of critical zones. The fatigue resistances of various structures by altering angles between the main barrel and up brace, and a novel structure inspired by topology optimization, are promptly verified. All results indicate that the present approach can determine the fatigue damage for the tripod structure with precision and efficiency.